This invention relates generally to fourth generation computed tomography (CT) systems, and more particularly, to methods and apparatus for aligning fourth generation CT systems.
Electron Beam Computed Tomography (EBCT) or fourth generation Computed Tomography is a non-invasive imaging technique for acquiring images of human organs. To acquire images using a CT scanner, a source, for example, a focused electron beam is directed toward a target, for example, one or more tungsten targets. The focusing is typically performed using a set of coils. As the electron beam hits the tungsten targets, X-rays are produced. These X-rays are directed at a ring of detectors. In operation, the X-rays pass through the ring of collimators, the object that is to be imaged, and are then detected by the ring of detectors.
In a fourth generation CT system the tungsten targets, rings of collimators, and ring of detectors are positioned in different planes. Alignment between the ring of detectors, ring of collimators and the tungsten targets is needed as the position of the ring of detectors and the alignment of the ring of detectors with respect to the tungsten targets defines a cone angle of the CT scanner. If the ring of collimators, ring of detectors and the tungsten targets are not aligned, the cone angle will be different at different azimuthal angles. The difference in the cone angle may result in X-rays that originate at points diametrically opposite to each other not intersecting at the center of the scanner. If the X-rays that originate at points diametrically opposite to each other do not intersect at the center of the scanner, artifacts may be produced in an image.
Further, if the X-rays that originate at points diametrically opposite to each other do not intersect at the center of the scanner, the thickness of the object scanned in the z-direction in one scan cycle of the scanner, also called the slice profile, will vary non-uniformly from the isocenter of the scanner. In addition to the alignment, positioning of the detectors and the collimators in the z-direction relative to the targets is also needed. Non-alignment of the detectors in the z-direction relative to the targets causes the cone angle to be higher than the designed optimum cone angle for the CT system. Higher cone angle results in a greater amount of cone artifacts, for example, shading under the body parts to be scanned, such as ribs. These artifacts are difficult to correct during the reconstruction of an image.
Furthermore, any non-alignment of the ring of collimators and the ring of detectors can cause shading at the detectors. The result of the shading is that some of the X-ray dose that is given to the patient will not be detected, and therefore will not be used to generate an image.
In currently known methods for alignment of fourth generation CT systems, the process of alignment is performed manually using a cone phantom that is placed in the center of a scanner. The process is completely manual wherein an operator scans from a software service tool and then analyzes the data manually. The data analysis involves looking at sinograms and plots from the X-ray data generated from the scanning of the cone phantom, and making adjustments by moving the ring of collimators or the ring of detectors, for example, by using motors. The adjustment made by the operator is based on an estimate of the service engineer and are not predictive.
These known processes of alignment depend on the skill and the judgment of the operator and the service engineer. Thus, there is a higher likelihood of error, for example, because of user analysis, judgment and manual adjustment. Further, these processes are very time consuming.